30 DISPERSAL AND MIGRATION. [part i. 



act very powerfully in modifying tlie distributiou of fresh-water 

 fish. 



Sea fish would seem at first sight to have almost unlimited 

 means of dispersal, but this is far from being the case. Tempera- 

 ture forms a complete barrier to a large number of species, cold 

 water being essential to many, while others can only dwell in 

 the warmth of the tropics. Deep water is another barrier to 

 large numbers of species which are adapted to shores and 

 shallows ; and thus the Atlantic is quite as impassable a gulf 

 to most fishes as it is to birds. Many sea fishes migrate to a 

 limited extent for the purpose of depositing their spawn in 

 favourable situations. The herring, an inhabitant of the deep 

 sea, comes in shoals to our coast in the breeding season ; while 

 the salmon quits the northern seas and enters our rivers, mount- 

 ing upwards to the clear cold water near their sources to deposit 

 its eggs. Keeping in mind the essential fact that changes of 

 temperature and of depth are the main barriers to the dispersal 

 of fish, we shall find little difficulty in tracing the causes that 

 have determined their distribution. 



Means of Dispersal of Molliisca. — The marine, fresh-water, and 

 land mollusca are three groups whose powers of dispersal and 

 consequent distribution are very different, and must be separately 

 considered. The Pteropoday the lanthina, and other groups of 

 floating molluscs, drift about in mid-ocean, and their dispersal 

 is probably limited chiefly by temperature, but perhaps also by 

 the presence of enemies or the scarcity of proper food. The 

 univalve and bivalve mollusca, of which the whelk and the 

 cockle may be taken as types, move so slowly in their adult 

 state, that we should expect them to have an exceedingly limited 

 distribution; but the young of all these are free swimming 

 embryos, and they thus have a powerful means of dispersal, and 

 are carried by tides and currents so as ultimately to spread over 

 every shore and shoal that off'ers conditions favourable for their 

 development. The fresh water molluscs, which one might at 

 first suppose could not range beyond their own river-basin, are 

 yet very widely distributed in common with almost all other 

 fresh water productions ; and Mr. Darwin has shown that this is 



